In known MOSFET structures, it is presently preferred to minimize the gate voltage VGS required for switching of the device and which then implies a relatively large input gate capacitance.
Capacitance inherent in the gate structures of insulated gate devices limits the switching speeds of these devices. It is also well known that the Miller effect has an influence on the input capacitance at the gate of devices of the aforementioned kind in that the input capacitance of a typical commercially available MOSFET varies during switching of the device. The input capacitance has a first value Ciiss when the device is off and a second value Cfiss when the device is on. The ratio of the second and first values for a known and commercially available IRF 740 power MOSFET is in the order of 2.5. It has been found that such a ratio impairs the switching speed of these devices.
The total switching time Ts of the IRF 740 MOSFET to switch on is made up by the sum of a turn-on delay time Tdon of about 14 ns and a drain source voltage fall time Tf of about 24 ns and is equal to about 38 ns. The corresponding time to switch off is about 77 ns. These times are too long for some applications.